The present invention generally relates to the production of concentrated hop materials, and more particularly to the production of a purified, iso-alpha-acid concentrate suitable for use in the production of brewed beverages.
The production of beer and other brewed beverages has traditionally involved the addition of hops and hop derivatives thereto. In particular, hop materials impart a distinctive, bitter flavor to brewed beverages. As described in Verzele, M., et al., Developments in Food Science, Vol. 27, (Elsevier Co., Amsterdam 1991), pp. 1-51, hop plants are in the Cannabinaceae family. Hop materials used in the production of beer and other brewed beverages are primarily derived from a plant in the genus Humulus known as Humulus lupulus L. The key flavoring ingredients obtained from hop plants reside within cone-like structures which are harvested and used in manufacturing brewed beverages. The primary flavoring ingredients in hop cones involve materials known as "alpha-acids". These alpha-acids shall be defined herein to encompass three primary compositions as follows: ##STR1##
Other humulones derived from hops which are encompassed within the term "alpha acids" include the following materials in lesser amounts: ##STR2## Of the total amount of alpha-acids present, tests have indicated that the foregoing ingredients are present in the following approximate proportions (% by weight): Humulone (35-70%), Cohumulone (20-65%), Adhumulone (10-15%), Prehumulone (1-10%), and Posthumulone (1-3%).
To be used in the production of brewed beverages as flavoring agents, the foregoing alpha-acids must be isomerized to form derivatives known as "iso-alpha-acids" The term "iso-alpha-acids" shall encompass the cis and trans isomerized forms of the humulones listed above, with the trans forms illustrated as follows: ##STR3##
There are numerous methods in which the isomerization of alpha-acids in hop materials (e.g. cones and extracts) may be achieved. For example, in the brewing of beer and other malt beverages, the addition of hops and/or hop extracts to the brew kettle, followed by the boiling thereof will result in the conversion of alpha-acids to iso-alpha-acids. However, this procedure is relatively inefficient and will typically result in the isomerization of only about 1/5 or less of the alpha-acids to iso-alpha-acids. Other processes involve the boiling of hop materials (e.g. hops or hop extracts) in highly alkaline solutions (e.g. NaOH). When this process is used, care must be taken to avoid pH levels above about 9.0 since degradation of the resulting iso-alpha-acids may take place at such levels.
Another method for the production of iso-alpha-acids involves the use of metallic isomerization materials. In particular, such materials are described in Verzele, M., et al., Developments in Food Science, Vol. 27, supra (pp. 109-111), and in U.S. Pat. No. 5,015,491 to Westwood et al. With reference to U.S. Pat. No. 5,015,491, a process is described wherein a hop extract is combined with a solid alkaline or alkaline earth metal salt (e.g. MgO) at a temperature of at least about 80.degree. C. In one embodiment of Westwood et al., a solid isohumulone mass is formed which may be added to the boiling contents of the brew kettle during the production of beer and other brewed beverages. Alternatively, the solid mass may be purified using a sequential acid/base treatment procedure conducted at sub-boiling temperatures. The use of metal compositions for the isomerization of alpha-acids is likewise discussed in U.S. Pat. No. 3,956,513 to Clarke et al.
While alpha-acids are effectively isomerized to iso-alpha-acids through the use of metal salt compositions, the use of metal-isomerized iso-alpha-acids in brewed beverages normally introduces metal ions into the production process. These metal ions (especially Mg, Ca, and Zn ions) can cause numerous problems in the final product, including but not limited to a hazy appearance and an undesirable flavor. In addition, excessive amounts of Fe and Cu ions can cause the generation of molecular oxygen in the final product which causes a corresponding deterioration in flavor and palatability. Thus, in a metal-catalyzed alpha-acid isomerization process, it is important to minimize the amount of metal compositions being used, and likewise ensure that the final isomerized product (e.g. the resulting iso-alpha-acid materials) have little or no residual metal ions associated therewith. In addition to achieving the foregoing goals, it is also important that reaction conditions be precisely controlled so that the isomerization process is driven to completion, thereby ensuring maximum economy and efficiency.
The present invention involves a unique and efficient method for generating a highly purified supply of iso-alpha-acids using metallic isomerizing agents which substantially minimizes the presence of extraneous metal ions in the final product (e.g. residual metal ions derived from the isomerizing agent). The invention likewise enables the isomerization process to occur in a substantially complete manner, thereby ensuring that the starting materials in the process (e.g. hop extract) are used in the most efficient manner possible.
Finally, the process described herein enables the production of a purified, concentrated supply of iso-alpha-acid materials which is not only substantially free from residual metal ions, but likewise includes minimal amounts of (1) terpene-based oil materials normally present in hops (hereinafter "hop oils"); and (2) materials known as beta-acids which primarily involve two compositions known as "lupulone" and "colupulone". Beta-acids are discussed in detail in Verzele, M., et al., Developments in Food Science, Vol. 27, supra (pp. 201-215), and are well known with respect to their characteristics and structural configurations. Both beta-acids and hop oils have unique aromatic and flavoring characteristics which may not be necessary or desired in brewed beverage products. Because the present invention accomplishes all of the foregoing goals, it represents an advance in the art of hop processing, and satisfies the need for a method in which iso-alpha-acid concentrate compositions may be produced with a maximum degree of purity and efficiency.